Make tempname more random, via the randint module.

Make tempname more random, via the randint module.

* gl/modules/tempname (Depends-on): Add randint and stdbool.
* gl/lib/tempname.c: Include randint.h and stdbool.h.
(uint64_t): Remove definition.  Not needed.
[_LIBC] (RANDOM_BITS): Remove this block, now that we have proper random bits.
(check_x_suffix): New function.
(gen_tempname_len): Rename from __gen_tempname.
Add a parameter, x_suffix_len, telling how many X's there must be at
the end of the template.
Use pseudo-random numbers all the way, rather than adding 7777
from one iteration to the next.
(__gen_tempname): New function, to call gen_tempname_len, requiring a
suffix length of 6.
* gl/lib/tempname.h: Add prototype for gen_tempname_len.

Signed-off-by: Jim Meyering <meyering@redhat.com>